1. Field of the Invention
The present invention is broadly concerned with novel compositions and methods of using those compositions to form protective layers that can protect an underlying wafer during acid etching and other harsh processing conditions. Such protection is useful in the manufacture of microelectronic devices such as those used in microelectromechanical systems (MEMS)
2. Description of the Prior Art
Deep silicon etching is an essential manufacturing step for all microelectromechanical systems (MEMS). Wet chemical etching in alkaline solutions has dimensional limits due to the differential etch rate of the crystal planes of silicon. Deep reactive etching (DRIE) processes have the ability to etch smaller features with high aspect ratios and can be used for new applications, such as the formation of through-silicon vias (TSVs) for three-dimensional (3-D) integrated circuit fabrication. This technique, however, requires expensive tooling and relatively long process times to complete the manufacturing cycle and still is deficient in meeting dimensional tolerances. New materials and processes are therefore needed to create very-high-aspect-ratio (VHAR), 3-D device features in silicon in a cost-efficient manner and with greater and more precise depth-to-width capabilities than current state-of-art technologies.
Photoelectrochemical (PEC) silicon etching utilize hydrofluoric acid (HF) as the active species to yield a highly controllable and high aspect ratio of greater than 120:1 for standing beams and trench structures within the silicon substrates. PEC deep silicon etching uses a low concentration (2-5%) of HF in water as the etching media, while oxide etching processes usually rely on concentrated aqueous HF (49%) or HF vapor (100%) to achieve suitable etching rates. In either case, masking layers are required to allow selective etching of different regions of the silicon device and/or to protect sensitive areas of the device from the corrosive effects of the etchant. Deposited layers such as silicon nitride, polysilicon, or even a metal mask have been traditionally used for this purpose. However, the need to deposit these layers using chemical vapor deposition (CVD), patterning them, and removing them creates great process flow complexity, which is very expensive and leads to high unit costs.
MEMS devices are increasing in complexity and are finding numerous applications in industrial and consumer products such as cellular phones, micromirrors, radio frequency (RF) devices, microprobes, and pressure sensors. One of the critical processing steps for these devices is release etching. In this step, a sacrificial layer, usually silicon oxide, is removed from certain regions to allow a range or motion of specific features. The thickness of the materials to be removed may vary from a few hundred angstroms to several microns. Because this sacrificial layer is silicon oxide in most cases. MEMS release etching has been historically performed using wet fluorinated chemistries that tend to produce strong surface tension and lead to stiction, resulting in either device malfunction or a reduction in the final product yield.
Recently, it has been demonstrated that using HF vapor for release etching can efficiently circumvent the stiction phenomenon because it substantially eliminates the surface tension that causes the stiction. During HF vapor etching, it is necessary to use masking or protective materials to protect the silicon oxide and metal features from HF attack. Traditionally, inorganic-based films such as silicon nitride (Si3N4), alumina (Al2O3), SiC, polysilicon, and aluminum were used to provide protection during HF vapor etching, but the effectiveness of their protection against HF attack was very limited due to the nature of the materials. Moreover, such inorganic masking layers require high-temperature deposition techniques, which are often lengthy and complicated. Films have been reported that can survive HF vapor etching processes for only 80 seconds or less, but this limits its practical applications. Some films deposited by chemical vapor deposition (CVD) can supposedly survive HF vapor etching for longer than 80 seconds, but CVD is an expensive and involved process.
Therefore, the demands placed on newly developed polymeric protective materials are very high. To date, however, there has been little success in using conventional photoresists or other common organic layers for HF etching because the very small HF molecules (˜0.92 Å in diameter) diffuse through, or in most cases decompose, these protective materials, thus leading to substrate corrosion, etching in protected areas, undercutting, and/or lifting of the masking layer from the edge.
There is a need for new polymeric protective coatings that are resistant to acid etching.